A substantial amount of salt is typically required for aluminum scrap remelting (recycling). Once used, the salt-containing residue is known as aluminum dross or saltcake. Saltcake is regarded as hazardous waste and therefore cannot be put into a landfill. Saltcake is normally sent, at further cost, to a recycling plant, where the salt is extracted and cleaned back to near its original specification, so that it can be used again.
In the process of remelting aluminum scrap in secondary aluminum production, the addition of a significant amount of salt means that salt can account for a substantial portion of the overall weight of the charged material. The material charged into the furnace consists of mainly aluminum scrap and salt. There are also additional materials that are purposely added to the mixture, depending on specific product requirements. For example, there are many different types of aluminum scrap which vary in composition and can contain various contaminants. For the purposes of this disclosure, the contaminants will be described as metal impurities (e.g., Mg, Si, Ca, Zn, Mn), oxides (e.g., MgO, SiO2, Al2O3), and organics (e.g., hydrocarbons, plastics, paints, coatings). Types of scrap can vary considerably, where new/clean scrap is considered to have more than 95% aluminum and any scrap with more than 5% contaminants is old/dirty scrap. Some scrap contains significantly more contaminants than others, such as coated packaging, where more than 20% of the material can consist of contaminants. Contaminants such as organics are removed during an initial stage of the melting process (i.e., the organics combust at low temperatures while the scrap is being heated).
Aluminum has a high affinity for oxygen and quickly forms a thin oxide layer when exposed to an oxidizing atmosphere. Therefore, all scrap has some percentage of aluminum oxide present from the onset of recycling. The aluminum oxide shell has a much higher melting point than the aluminum and therefore does not melt inside an aluminum recycling furnace. The aluminum oxide shell must be chemically or mechanically broken, allowing the molten aluminum to escape. Subsequently, the less dense oxide material floats to the surface. If the molten aluminum is not protected from the oxidizing atmosphere inside the furnace, it will undergo further oxidation, reducing yield. The formation of the additional aluminum oxide acts like a net, trapping molten aluminum within its structure, also reducing yield.
Salt is added to the furnace in order to improve the melting process and can have a number of benefits. Typically, the mass of salt added to the aluminum scrap in the furnace is from about 5% to about 15% of the mass of the aluminum scrap, depending on the type of scrap, the type of furnace, the operating methodology, and several other parameters. The main duty of salt is to protect the aluminum from the oxidizing atmosphere. Salt also partakes in the reaction by providing a chemical mechanism for breaking up the aluminum oxide shell of the scrap. The salt aids in breaking up the aluminum oxide formed during the melt, releasing some of the entrapped aluminum. Mechanical stirrers or rotary furnaces are often used for aiding the breaking up of aluminum oxide. Salt also reacts with metal impurities to aid in removing them. Other benefits of salt include changing the melt properties, such as density and viscosity, improving the separation between the melt and its contaminants.
Different types of aluminum melting furnaces have been devised to reduce the amount of salt used in the recycling process. However, those that eliminate a significant amount of the salt are much less efficient and therefore are not the ideal solution. There is a need in the industry for providing significant cost savings by reducing salt usage without causing significant detriment to the cycle efficiency, yield, or cost.
An earlier patent, U.S. Pat. No. 5,563,903, describes a method of introducing a single non-oxidizing (protective) layer or stratum of gas into an aluminum recycling furnace between the combustion zone or strata and the aluminum, in order to reduce oxidative attack on the aluminum and to thereby decrease dross formation and increase yield. This scheme is shown generally in FIG. 1. A non-oxidizing layer 101 may comprise an inert gas such as nitrogen or argon, or a reducing gas such as hydrogen, methane, or other hydrocarbon. In this implementation, both combustion reactants 104 (for example natural gas and air/oxygen) and a non-oxidizing gas 105 (for example nitrogen) are introduced at low velocities into the furnace, in order to minimize mixing of the two strata of gases 101 and 103. Specifically, a low velocity burner, either a laminar flow burner or a premixed radiant-type burner, is recommended to reduce mixing between the combustion layer 103 and the non-oxidizing layer 101, and the velocity of the non-oxidizing gas 105 is taught not to exceed 50 feet per second, and preferably to be less than 20 feet per second. There is no reduction in salt consumption.
Other attempts have been made to reduce oxidation of the metal, for example by using an oxidant-staged burner that forms a fuel-rich (reducing) flame near the metal surface and a stoichiometric or fuel-lean flame on the opposite side of the fuel-rich flame from the metal bath. See, for example, U.S. Pat. No. 8,806,897, albeit relating to glass furnaces. Similarly, a system such as described in EP 0962540 provides an oxygen lance above an air-fuel burner, which is operated with a sub-stoichiometric amount of air (i.e., fuel-rich). The oxygen lance must be installed above the burner, so that the reducing atmosphere of the fuel-rich burner acts as the barrier between the oxidizing flow and the aluminum.
In addition, other systems employ a similar idea to effectively create a non-oxidizing or reducing layer by operating a standard tube-in-tube (two concentric tubes or pipes) burner with oxidizer flowing through the central tube and fuel flowing through the annular space between the tubes. This inhibits oxygen contact with the melt because it is used up in the combustion zone. While such an arrangement tends to reduce the oxygen escaping from the burner, it does nothing actively to protect the molten aluminum from those free oxygen molecules that do escape.